Well-bore zonal isolation is a very important requirement for both geological storage of CO2, and oil and gas production. It is a prerequisite for efficient and safe operation. Presence of micro-annuli, isolation defects or poor quality cementing facilitates hydraulic communication, thus allowing fluid migration, and pose a safety and contamination risk. Lack of proper isolation leads to costly treatment facilities, well intervention and operational interruptions. Isolation is achieved by pumping cement through the annulus between the casing and the formation.
In CO2 sequestration and oil and gas wells, estimating the quality of the annular isolation and repairing the cement where necessary is important for preventing potential leaks and fluid contamination. The presence of a mudcake adjacent to the formation and the lack of slip at the walls may lead to unfilled annuli during cementing. Cracks and micro-annuli may also form during setting and shrinkage, and radial cracks may be initiated due to expansion of the casing during pressurization. Such imperfections in cement facilitate inter-zonal migration. Additionally chemical alteration of cement is also complex and depends on thermodynamics, kinetics and diffusion of reactive species leading to reaction fronts. Both mechanical and chemical processes can cause radial and azimuthal variations in the cement properties. The ability to detect the presence of micro-annuli or isolation defects and where possible, quantitatively estimate cement transmissibility is crucial for ensuring project safety.
The quality of cement in the annulus is traditionally evaluated by ultrasonic measurements. These measurements, however, provide only qualitative evaluation of hydraulic isolation and are not suitable for volumetric estimation of subtle cement defects or cement transmissibility.
Other work has focused on techniques to quantify cement permeability in the annulus. These methods are based on the relationship between the observed pressure and the flow rate through a pressure probe set behind the casing. The flow rate may in turn be expressed in terms of the decompression characteristics of the fluid in the tool. Elimination of the flow rate allows one to obtain an explicit expression for the pressure decay in terms of permeability in the local region around the probe. Significant variation in the permeability estimates obtained at progressive depths of probe penetration within the cemented annulus could be interpreted as an indication of the cement permeability alteration. Although these procedures could be used to detect the changes in hydraulic isolation of the cement sheath, they do not provide any information on the presence and size of the isolation defects—a key input into the remedial action plan.